The invention relates to improving the performance of USSD (Unstructured Supplementary Service Data) transfer in a cellular communications system, such as GSM (Global System for Mobile Communication).
The user of a mobile station (MS) can use USSD to give instructions to the supporting PLMN (Public Land based Mobile Network). For example, incoming calls can be routed to number 123456 by dialling *21*#123456#. USSD is also one of the mechanisms for implementing new services. USSD allows an MS and a service application to communicate with each other by character strings, in a way which is transparent to the MS and to the intermediate network elements. USSD can be used as a narrow-band bearer for over-the-air (OTA) and value-added services (VAS) applications. With respect to a more detailed description of the USSD, reference is made to the following ETSI GSM recommendations: GSM 02.90: European digital cellular telecommunications system (Phase 2); Stage 1 description of Unstructured Supplementary Service Data (USSD), GSM 03.90: Digital cellular telecommunications system (Phase 2); Unstructured Supplementary Service Data (USSD)xe2x80x94Stage 2, and GSM 04.90: European digital cellular telecommunications system (Phase 2); Unstructured Supplementary Service Data (USSD)xe2x80x94Stage 3. USSD requests, notifications and responses contain a USSD string, an alphabet indicator and a language indicator, as defined in GSM 03.38.
USSD signalling may be initiated by the mobile station or by the network. Phase 1 supports only MS-initiated USSD. Network-initiated USSD service requires that all parts of the mobile communications system be at least phase 2 systems. The mobile communications network may at any time send a USSD message to a mobile station MS registered with the network in order to transmit information to the subscriber. This operation may be either a request (asking the MS to provide information) or a notification (requiring no information to be provided by the MS). No prior provisioning of USSD is required, although provisioning of services which make use of USSD may be required.
According to the above ETSI recommendations, USSD signalling takes place between an MS and an MSC/VLR (Mobile services Switching Centre/Visitor Location Register) or HLR (Home Location Register). USSD supports a maximum of 160 bytes of user data per message. (The upper limit can be less than 160 bytes depending on the underlying protocol layers.) Unlike SMS (Short Message Service), USSD has no store-and-forward functionality: mobile-terminated USSD messages are delivered to the MS immediately, or the delivery fails (e.g. because the MS is unreachable).
For the purposes of this application, a GSM-type mobile station has two modes: a call mode and an idle mode. A mobile station is in call mode if and only if it is xe2x80x9cin a callxe2x80x9d, which state is defined in the GSM recommendation 02.30.
According to the above ETSI recommendations, USSD transfer takes place on two different channels depending on whether or not the MS is in call mode or idle mode. In call mode, Fast Associated Control Channel (FACCH) is used. In idle mode, Slow Dedicated Control Channel (SDCCH) is used.
The speed of the FACCH channel is approximately 140 bytes per second and that of the SDCCH channel approximately 83 bytes per second. Thus, even in idle mode, any USSD message can be delivered in less than two seconds.
It is conceivable that the use of USSD for implementing value-added and over-the-air services will increase. In this case, especially if multiple consecutive USSD messages are needed, the slow transfer speed of the SDCCH channel could be seen as a problem. Also, prolonged use of the FACCH channel degrades the quality of speech on the associated speech channel. (It should be noted that for keeping the description compact, FACCH is used as a synonym for the fast channel, and SDCCH is used as a synonym for the slow channel. However, FACCH and SDCCH are terms used in the GSM system and its derivatives, but these terms are not necessarily used in future cellular systems.)
Accordingly, it is an object of the present invention to study if the speed of USSD in idle mode could be improved, and if yes, to provide a method and equipment for improving the speed of USSD in idle mode. Another object of the present invention is to study if the degradation in speech quality due to prolonged use of the FACCH channel can be minimised. Yet another object is to develop a mechanism that is expandable to accommodate future modifications. All these objects are achieved with a method and equipment which are characterized by what is disclosed in the attached independent claims. Preferred embodiments are disclosed in the attached dependent claims.
A straightforward way of improving the speed of USSD transfer would be to specify that all USSD transfers take place on the fast FACCH channel. This would, however, require changes in existing standardisation. Also, the FACCH is not a dedicated channel, but an associated one, which means that it is implemented by stealing bits from the associated speech channel, if one exists. If such bit stealing is allowed to go on frequently and for long periods of time, it will degrade speech quality to some extent. On the other hand, if the mobile station is not in call mode, establishing the FACCH for the sole purpose of USSD transfer requires establishing a corresponding speech channel, which could be seen as a waste of radio resources. The user of the MS may not want to pay extra for speeding up the USSD transfer and, if the network is heavily loaded, the operator may not want to allocate an extra speech channel for the sole purpose of speeding up USSD transfer.
Similarly, a straightforward way of eliminating the degradation of speech quality would be to specify that all USSD transfers take place on the slow SDCCH channel. This would, however, be a drastic solution. Moreover, in the current GSM system an SDCCH channel does not exist when the MS is in call mode.
The invention is based on locating the problem and finding a solution for it. The solution is based on an expandable concept that can be implemented, at first, as a simple on/off mechanism, but which can be extended to balance the different needs and interests of the parties of the call as listeners, the MS user as a user of OTA and/or VAS applications, the network operator, and the other users of the network.
According to the invention, the degradation of speech quality is lessened or totally eliminated by substantially lowering the data rate of the USSD transfer when necessary, in order to maintain adequate speech quality.
The data rate can be lowered by using the dedicated control channel SDCCH, if one exists. In a GSM system and its derivatives, the SDCCH exists unless the mobile station is in call mode. If a dedicated control channel does not exist, the data rate can be lowered by dividing a long USSD transfer into several consecutive USSD messages and delaying individual USSD messages (by inserting gaps between them) until the degradation of speech quality is brought to an acceptable level.
The expandable concept for performing USSD transfer can be implemented e.g. by a method including at least the following steps:
(i) evaluating a set of factors in favour of the maximum data rate for forming a totality of the factors, the totality being at least partly independent of the existence of said associated channel prior to said evaluating; and
(ii) using the totality of the factors to determine whether to perform the USSD transfer substantially at the maximum data rate or at a lowered data rate.
Performing the USSD transfer substantially at the maximum data rate implies that the FACCH channel will be used and that the data rate is not lowered sufficiently to substantially improve the quality of possible simultaneous speech on the speech channel (the improvement due to the lowered data rate is not clearly audible to a human ear).
In this context, the totality of factors means more or less the same as evaluating substantially all factors available to (i.e. known by) the decision-making process. By giving different interpretations to the xe2x80x98factors in favour of the maximum data ratexe2x80x99 the invention can be implemented in an extendible manner.
The invention can be implemented in a very simple manner if the totality of factors favouring the maximum data rate is a simple determination that a need for USSD transfer exists and an FACCH channel is available or can be established.
The totality of the factors in favour of the maximum data rate can be used to determine the data rate by comparing the totality of the factors to a predetermined margin and by performing the USSD transfer substantially at the maximum data rate in response to a positive result in the comparison. If the result of the comparison is negative, the USSD transfer takes place at a lower data rate. Preferably, this means performing the USSD transfer on a dedicated control channel (SDCCH), if one exists, and otherwise delaying individual USSD messages. The network operator can transmit to decision-making units, such as mobile stations, one or more elements affecting the comparison, such as the predetermined margin. For example, when the network load increases, the operator can transmit a higher margin which would disfavour the use of the maximum data rate. Such parameter(s) can be transmitted on a broadcast control channel, or as USSD or short message transmissions, as is well known to those skilled in the art. Because the predetermined margin is compared with the factors favouring the maximum data rate, the same result is achieved if the network operator transmits one or more elements affecting these factors, as will be described later in more detail.
A more flexible implementation than a simple on/off approach is achieved if the totality of the factors is formed by weighing each factor with a respective weight and combining the results of the weighing. It is also possible to similarly evaluate another set of second (negative) factors disfavouring the maximum data rate. Such second, or negative, factors could include a determination that the network is heavily loaded. However, this is not necessary, considering that all such negative factors have an opposite positive factor (i.e. an absence of a negative factor). For example, the opposite of a heavy load in the network is the availability of radio resources.
Positive factors favouring the maximum data rate could comprise one or more of the following:
1) the amount of data to be transmitted;
2) the availability of radio resources in the network;
3) the relative amount of silence on the speech channel;
4a) the length of time from the beginning of the call
4b) duration of continuous speech, i.e. the length of time from the latest period of silence on the associated speech channel
5) the MS user""s preference of USSD transfer over speech quality.
The availability of radio resources in the network should be considered, especially when there is no call going on between the mobile station and the network. In such a case, USSD transfer is the only reason for establishing a speech channel, which is necessary for establishing an FACCH. In a heavily loaded network, the operator can be reluctant to allocate a speech channel for the sole purpose of speeding up USSD transfer. On the other hand, if there is a call going on, then using the FACCH does not consume any additional resources, but it does degrade the quality of speech to some extent.
The amount of data to be transmitted should be considered too. There is little point in using the FACCH for very short transmissions.
The relative amount of silence on the speech channel could be considered as well. Using the FACCH degrades the quality of speech to some extent but this is of no consequence if there is currently no speech on the speech channel. Considering that a call involves two parties, of which usually only one speaks at any given moment, approximately 50 per cent of the time the content of the speech channel does not consist of speech frames but silence indicators, known as SID (Silence Descriptor) frames in the GSM system. A base station subsystem BSS (a base station, base station controller and/or a transcoder) could be modified to respond to an inquiry from an external node regarding the relative amount of SID frames. It is to be expected that in future cellular systems, such as UMTS (Universal Mobile Telecommunications System), the elements of the BSS will be more deeply integrated into a Radio Network Controller RNC. Such integration facilitates implementing the invention in respect of detecting silence indicators on the speech channel.
A small relative weight could also be given to the length of time from the beginning of the call and/or from the latest period of silence on the speech channel. This is because the beginnings of continuous speech are crucial for understanding. If the call has been going on for a minute or two and/or continuous speech has been going on for a few seconds, the listener can much better tolerate the degradation of speech quality due to usage of the FACCH.
For the purpose of demonstrating the expandable nature of the invention, some additional factors will be presented near the end of the description.
An USSD transfer may consist of several separate USSD messages and it may go on for several minutes. (Currently, its maximum duration is limited to 10 minutes.) During a long USSD transfer, the factors favouring (or disfavouring) the use of the FACCH may change considerably. Thus it is preferable to repeat the evaluation between successive USSD messages, although not necessarily between every two messages.